In general, an anti-lock brake system (ABS) is a special brake that has been developed to prevent wheels from being locked at the time of quickly braking of a vehicle. The ABS continuously applies increased pressure and reduced pressure to the wheels, thereby providing stable braking and steering performance by preventing the vehicle from slipping at the time of braking. When describing an operational mechanism of the existing pressure reducing solenoid valve for increasing and reducing pressure of the ABS, high pressure is applied to a wheel cylinder side, and a plunger and a seat come into close contact with each other by hydraulic force and spring force that push down the plunger, such that oil cannot flow from the wheel cylinder side to an accumulator side. In this state, when an electrical signal is applied to a coil in order to reduce pressure, the plunger is moved upward by electromagnetic force, such that the plunger and the seat are moved away from each other. Accordingly, the oil passes through a portion between the plunger and the seat from the wheel cylinder side, and then flows toward the accumulator side. Therefore, high pressure at the wheel cylinder side is reduced, and pressure at the vehicle wheel side is reduced, thereby unlocking the brake.
In contrast, at normal times, hydraulic pressure is applied to the wheel cylinder side, and the plunger and the seat come into close contact with each other by hydraulic force and spring force that push down the plunger, such that the oil does not flow from the wheel cylinder side to the accumulator side.
Subsequently, in the case of the existing pressure reducing solenoid valve, when a direction of electromagnetic force, which is generated when an electrical signal is applied to the coil, is opposite to a direction of hydraulic force, it is impossible to accurately control pressure of the wheel, such that an operation of reducing pressure is performed in an on-off control manner.
Therefore, there is a problem in that noise is generated at the time of controlling the ABS or an ESC, and a brake pedal is not normally operated, such that comfort of the vehicle deteriorates. As described above, the existing pressure valve has a limitation in satisfying needs for a urban smart cruise control (SCC) for the vehicle and the implementation of additional functions using various low pressure.
For example, Korean Patent Application Laid-Open No. 10-2011-0061693 discloses “Solenoid Valve Assembly for Preventing Drive-Slip of Vehicle”.